In recent years, there has been an increasing interest in the use of ceramic materials as structural components in environments historically dominated by metallic members. The impetus for this interest has been the superiority of ceramics when compared with metals with respect to certain properties such as corrosion resistance, hardness, modulus of elasticity, thermal insulation or conduction properties, and refractory capabilities. As a consequence, ceramics have been selected, or are under development, for use as engine components, heat exchangers, cutting tools, bearings and wear surfaces, pumps and marine hardware.
From among the enumerated fields of utility, the integration of ceramic materials into internal combustion engines offers a significant opportunity for functional improvement and resultant efficiency. Ceramics have lower coefficients of friction than metals contributing to mechanical properties which are superior to those of mating metals, which is true even in the high temperature environment of an internal combustion engine. Ceramics also exhibit more favorable characteristics respecting dimensional stability over the wide temperature ranges encountered. Certain ceramics are thermal insulators when compared with metals, and this too provides an opportunity for improved engine efficiencies since a large proportion (reportedly as high as about 75%) of the energy of fuel can be lost as waste heat. The goal of designing an adiabatic internal combustion engine employing ceramics, with their low thermal conductivity but good dimensional stability, becomes close to realization in efforts to utilize more completely the energy value of the combusted fuel. Accordingly, advances in ceramic material development are being driven by long-felt pragmatic needs.
Other properties of ceramic components, however, have limited their full involvement in certain hostile environments calling for members which possess good structural integrity, with reduced tendencies toward wear from erosion or cavitation. Of course, fully densified ceramic materials may meet some of these requirements; however, manufacturing fully dense ceramics, and particularly those having any complexity of shape, is difficult when low-cost reproducibility part-to-part in mass production are also requirements.
Limited success has been attained in efforts to manufacture dense ceramic components, whether as an integral article or as a composite. One such approach is disclosed in U.S. Pat. No. 3,437,468 to Seufert. This patent relates to alumina/spinel ceramic composites. The process disclosed includes establishing a molten pool of aluminum covered with a thick layer of finely divided magnesium silicate particulate. Molten aluminum is transported through the particulate layer wherein it is partially oxidized via the oxidation-reduction of aluminum and magnesium silicate as well as oxidation via atmospheric oxygen. The process ultimately yields a composite of multiple oxide phases and metal phases; namely, spinel, alpha-alumina, free silicon and/or a silicon-aluminum phase reported as an intermetallic, usually also including elemental or free aluminum. The reaction tends to be slow, and oxidation is promoted such as by means of an alkali metal oxide. The product is recovered and ground to a desired particle size, and then mixed with suitable resins to form molded, high-friction articles.
There have been other attempts to produce ceramic structures more nearly approximating the net shape of the desired article by using particulate precursor metal and air oxidation. U.S. Pat. No. 3,255,027 to Talsma, U.S. Pat. No. 3,473,938 to Oberlin, and U.S. Pat. No. 3,473,987 to Sowards disclose such processes for making integral skeletal structures, e.g. honeycomb. In the '027 patent, particulate aluminum or aluminum alloy is combined with a metallic oxide fluxing agent, and also may include a particulate filler refractory. The admixture is oxidized to convert the aluminum to corundum. A generally porous structure is developed having intrinsically low strength properties.
The invention of the '027 patent is characterized by its assignee as producing a structure having an interior void. The '938 patent proposes to overcome this drawback by incorporating within the initial mix a vanadium compound which, under process conditions, causes the formation of alumina bridges. The '987 patent also purports to improve the strength of the double-walled alumina product of Talsma by the in situ oxidation of aluminum templates if the templates are coated with aluminum powder, a fluxing agent, and a refractory filler.
U.S. Pat. No. 3,298,842 to Seufert discloses a method for forming hollow refractory particles such as hollow alumina particles. A porous admixture of aluminum particles or aluminum alloy particles, a refractory diluent, and a catalyst is heated in air to a temperature above about 650.degree. C. but below that at which self-bonding or sintering of the refractory diluent would occur. The metallic particles oxidize at their surfaces, consuming the free metal and leaving behind a void to yield the hollow refractory particles which are physically separated from the refractory diluent. The diluent may be inert or reactive within that process; when the former, it is present in an amount of at least five times the volume of the metallic constituent, in the latter it is present in an amount at least seven times that of the metallic component. These substantial proportions of diluent are necessary to minimize agglomeration of the oxidized particles and, accordingly, to avoid a continuously bonded structure which would render recovery of the desired alumina particles more difficult. Further along these lines, the admixture is fired in a relatively uncompacted form to reduce tendencies toward diluent bonding while promoting ready access of the oxidant to the aluminum of the admixture. Porosities of at least 60%, and more preferably 70%, are recommended by that patentee.
While much attention has been paid and considerable efforts devoted to the fabrication of ceramic articles, including the manufacture of ceramic articles by in situ oxidation of precursor metals, these previous attempts have been lacking in one or more respects with regard to the development of products having structural integrity rendering them suitable as articles of commerce. For example, the migration of aluminum from a foil configuration to develop a double-walled ceramic structure severely handicaps an article manufactured in that way from adaptation as a structural component for lack of strength, particularly compressive and/or flexural strengths. Certain of the fabrication techniques themselves are cumbersome, requiring repetitive coatings of templates or the like interspersed with drying steps.
A further deficiency of past approaches utilizing in situ oxidation of powders, foils and wires to create ceramic bodies has been the exceedingly poor contact wear and erosion resistance of such bodies. It is the inherent porosity of the products of the prior art which is responsible for their poor structural and wear properties and this has greatly limited any practical utility of such inventions.
Those approaches which oxidize a template and thus strive to attain that goal are limited to producing inherently weak structures. On the other hand, other efforts which have to date yielded ceramic structural components of acceptable strength have been limited to manufacturing processes requiring high pressures and temperatures in order to accomplish sintering and densification of their constituents, making their costs disadvantageous. Accordingly, the art, is, in the main, lacking in respect of efficient methods for producing useful structural articles.